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Can lift occur if air is flowing over an object, but not under it?
Can lift occur if air is flowing over a fixed-shape object, but no air is flowing under any part of the object? Maybe the object is the top half of a wing that's glued to the ground, or the object is dome-shaped, or some other shape. Maybe the object doesn’t actually rise or move, but when air flows over it, it exerts less weight on the ground.

Hurricanes and roofs!

@Charles Bretana: Though that could easily be wind getting under the edges of shingles &c, and forcing them up.

@CharlesBretana I have seen a "topper" fly off a pickup truck, it went about ten meters up.

All the answers here, including the one accepted by the OP, suffer from a misconception.  Lift comes from underneath because of the pressure differential. On a weighing scale, the wing is not 'glued to the ground' it is on a moveable platform. This platform is pushed upwards by the higher  pressure from underneath.  No-one has answered the 'stuck to the ground' question, where the base is immovable and not open to the air. I suggest that there is no lift in that case.

@chasly-supportsMonica How might one test that hypothesis?

@Camille Goudeseune - Good question. Strain gauges maybe?  You cannot use a hollow aerofoil because that will contain air. You would have to use a solid material.

However one measures "exerts less weight on the ground, or not," it'd have to deal with how almost any material, even a thick steel beam, deflects when force is applied.  Just like the millimillimeter that my scale deflected.  So, seal my scale in an airtight plastic bag?

@Camille Goudeseune - Your scale would still have air inside it. Maybe there are solid-state scales? Perhaps I'll ask this on Physics SE and see if anyone knows a method. :-)  P.S. I think I might have an idea but I'll have to think about it.  PPS - Just thought of another idea.

Yes.  This jar of paint weighs 34.8 g, but only 32.6 g to 33.4 g when a fan blows at it.  The reduction of weight persisted when I repeated the experiment with a hodgepodge of baffles made from CD jewel cases and Post-Its to prevent any air leakage under the scale's sensor plate.  Others can easily replicate these results.
Per FlanMan's comment, without the fan, I also tried pushing the jar horizontally with a very sharp pencil, like a pool cue to avoid imparting vertical force.
No matter where I put the jar on the scale,
no matter which way I pushed relative to the scale,
no matter how high the pencil tip touched the jar,
by the time the jar moved, the scale's report changed no more than -0.1 g to +0.2 g, and usually increased: it "got heavier" instead of "producing lift."
(The fan wasn't nearly strong enough to move the jar.  High volume, low pressure, in paint sprayer parlance.)
So the horizontal component of the fan's force accounts for at most 1/7 to 1/10 of the effect.  One could throw some statistics at this, or a milligram-accurate jeweler's scale, but more confidence would come from trying this in a proper wind tunnel whose load cell is quite independent of the air flow.
With baffles, with fan, without jar, the scale reported -0.5 g to +0.5 g, tending towards the negative (i.e., lift), depending on how I placed the baffles and the fan.  However one might label that effect, at least some of the time this flat plate also counted as an example that answers yes to the question.
The scale is a DigiWeigh DW-500BS, unsurprisingly out of production, capacity 500 g, claimed repeatability 0.1 g which I've confirmed every few months with the calibration weights from a triple beam balance.

Nice experiment! If you omit the fan and instead press against the jar horizontally with a pencil, can you get the scale to show less weight? If so, then possibly the air is only pushing the weight of the jar away from the scale's sensor, rather than creating lift.

one.   Could be the scale tray is generating lift (as per Coanda).  May we just tare the scale to 0.0 g without the paint (although the bottle, like a stone in the river, will also create lower pressure behind it as well)

Off topic, but what's the brand/model of that scale?  Amd to the question, if you have a tarp laid flat on the ground (or on your roof, or covering a load on a truck), it will flap around in the wind even if you take care to seal the edges down so the wind can't get underneath.

One slight problem here. There is air pressure underneath the pan. Therefore this is not 'glued to the ground'.

Yes.
Bernoulli end effectors are used in industrial applications to pick up items without physical contact using this principle.
Example:

Interesting! What's the advantage to a Bernoulli gripper over say a vacuum? Any idea?

@curious_cat Because a vacuum will pull the item into physical contact with the suction head.  For delicate items (like the silicon wafer shown) this might damage them.

Kodak had these installed decades ago to assist in moving large bars of silver for film production. Bars weighed in at about 70 lbs; the vacuum assist lifter suctioned to the top/bleed, the tube would compress/shrink, and the net weight was 0 (but still obviously a lot of mass). Pretty amazing to see in operation- and it expanded the pool of people available to do the work, creating more jobs.

@J.Hirsch That sounds fascinating! But I'm still trying to understand it from your description. Have any other links or sketches? Did not understand the 0 weight part nor the role of vacuum vs Bernoulli both in the same application.

@curious_cat https://www.youtube.com/watch?v=ZOTE5zU_uzY - quick search that looked like the item in use, but obviously wasn't because, cough 20 years ago... feeling old today

@J.Hirsch: The thing in this answer blows air out, not suction.  The video gets to this after about 25 sec.  What you're talking about, for heavy sturdy items, is suction that would result in far less uniform forces on the object, with a high-force area right under the vacuum head, and no force elsewhere.  That's fine for moving bars of silver, but not silicon wafers or pieces of cloth like shown in the video in this answer.

Whoops! You're right- my fault for not studying the item in question fast. I'm assuming they'd use N2 for the wafers then.

Yes. A classical example of this often taught in schools is the experiment where you cut a strip of paper and blow across the top of it, causing the paper to lift.  Here is a video demonstration:

Yes, because the still air under the object has (by Bernoulli) a higher pressure than the moving air above.
That's why, in high winds, entire roofs are sometimes detached from houses...

A higher static pressure, right?

I'm sure that's part of it, but the air flowing downward beyond the object also plays a large part.

@Max, exactly right! Total Pressure, (the vector sum of static and dynamic pressure), at least sub-sonically, must remain constant. It has to, because of conservation of energy.


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What could explain the fact that during 1976-80, I used to hear commercial jets make sonic booms while flying overhead?
I've read on this site & on Wikipedia that there have only been a couple of commercial jet planes in history that have broken the sound barrier--either in testing (without passengers) or during an unexpected descent.  However, during the period of 1976-80/81, when I lived fairly close to an international airport, I used to regularly hear commercial jets (I assume 747's) create sonic booms as they passed fairly low overhead. I now live even closer to a much bigger international airport, & never hear this. That's why I started to wonder why. & now that I've read that on record, it's only happened a couple of times, I'm even more curious of what the explanation could be. (It was Bradley Airport in CT, if that matters).

Hard to know what you actually heard but Bradley is also shared by the CT ANG. Military jets make all sorts of loud noises that aren’t common with commercial planes, maybe that’s part of the story?

Was the airport perhaps also used by the military or was there a nearby AF base?

If you can't tell the difference between what you heard and a gunshot or an explosion then it's a sonic boom. If it doesn't sound like a gunshot or explosion then it's not a sonic boom. If you really did hear a sonic boom perhaps you mistook it for a real gunshot?

When I was growing up in the 80s, we used to hear them from fighter jets going over head.  That stopped pretty soon after '87? '88?  they still happen off the coast; but not so much over land any more: https://www.wmbfnews.com/2020/02/28/neighbors-little-river-longs-area-report-homes-shaking-heres-what-we-know/

It's not quite true that commercial planes have only broken the sound barrier in testing or unexpected descents. There were many scheduled supersonic flights on Concorde - which actually started flying to Washington in 1976. But it was only allowed to go supersonic over the ocean, to avoid hitting people on the ground with sonic booms, so that probably wasn't what you heard. There was also a Soviet supersonic passenger jet, but that's probably not it either.

I've read that military aircraft often use the Death Valley National Park as a place to try low-altitude supersonic flight. One even crashed a few months ago while doing so.

@slebetman I'd compare it more to a (large) cannon shot than a normal gunshot. It's much louder than a normal gun, at least in my experience. Even from a significant distance, it can easily shake a building.

@user6030, please post a link or reference to where you heard this.  You can’t routinely fly supersonic in CONUS, let alone in National Parks.  I have done several Red Flag exercises operating in remote MOAs and it was a no-no even there.

@slebetman, there is a pretty big difference in sound between a gunshot and a sonic boom.

Opinion based and not a definitively answerable question

In the early-to-mid 1960's, the Air National Guard at Fort Wayne, Indiana's Baer Field did some low-level supersonic exercises on a regular basis. Living just a mile away (somewhat out-in-the-country), we saw the jets, and felt the sonic booms. So low-level supersonic flight was evidently permissible in those days, for military aircraft. And a sonic boom is unmistakeable, after you've heard such.

Two examples of the same sonic boom events from different distances: https://youtu.be/otfBviE1G3k at 1:40 and https://youtu.be/VBlIvghQTlI at 0:30. These are doubled as there are two rockets making the booms. Do these sound familiar?

All of the sonic booms I ever heard in the 60's, when they were fairly common in western Washington, made double or in some cases, triple booms.  They were easily distinguishable from gunshots and explosions, both of which were also quite common in my rural area at the time. When there were multiple super-sonic aircraft nearby, it was often indistinguishable from thunder.  A super-sonic rocket might produce a single boom, due it's envelope.  I suppose more modern planes might have only single booms.

@jwdonahue A WW2 German Panzer 88mm made a double report.

@RussellMcMahon, I don't doubt it. First you'd hear the round go over head, then you'd hear the muzzle blast that followed it.  For aircraft however, it appears the cause has to do with the leading and trailing edges of the aircraft: https://www.google.com/search?q=double+sonic+boom+from+aircraft.

@jwdonahue I understood it was a characteristic of the actual firing. Perhaps shell departing and pressure wave thereafter. Not mentioned here but it's interesting.

It sounded like an explosion.  & I believe Bradley shares their space with the National Airforce.  But I definitely remember these jets looking like normal commercial jets--very big & white.

Note that 747's and other jumbo jets operating out of Bradley could not have produced sonic booms because they do not fly above the speed of sound (they only do 500 to 550 MPH at high altitude cruise) and, in any case, far slower than that (~250 MPH) when near the ground as for landing and takeoff.
Therefore, whatever it was you heard, it was assuredly not a sonic boom from a passenger plane. The last time these commonly occurred was in the 1950's and '60s when the US Air Force flew mock bombing missions across the United States in supersonic bombers at low altitude, to train the pilots and crews for similar missions against targets in the USSR.
Although sonic booms were essentially outlawed over populated zones in the early 1960's, in your case they apparently did happen during flight tests of the F-22 Raptor in your location, which is very interesting! Thanks for @reirab's  detective work which uncovered this.

Comments are not for extended discussion; this conversation has been moved to chat.

Can you advise where the comment re F22s by Reirab comes from. I don't see his name or F22 mentioned on this page or in the now removed comments to your post. Sounds interesting.

@RussellMcMahon, he said it was originally posted & answered on the aviation stack exchange.

@nielsnielsen Reirab spoke of Tennessee & OP here is talking about Connecticut, I don't see how they could be talking about the same thing.

@nielsnielsen I searched the site but alas can find no mention. It sounds worth commenting on in more detail in an answer.

@RussellMcMahon, search for reirab under "users" (tab on left), you'll find him.

@Russell McMahon What I used to see was definitely not the F22!  Looked absolutely nothing like it!  It was big & white! I think it may have been the Concorde (See below).  Only thing that may not fit, according to comments, is that it was always going due SW.  (Apparently, it flew to Kennedy airport, but I suppose it may have not been able to fly directly in that direction for some reason).

@RussellMcMahon Sorry, just now saw your comment from 4 years ago. - haha - You can see my original comments in the "moved to chat" link in Farhan's comment above. This question discusses military supersonic flights in the U.S. This comment mentions when they were flight testing the F-22s over Arnold AFB (where I worked and also heard them) in '05-'08.

It was either not a sonic boom, or it was not a commercial jet. As Niels has pointed out, civilian aircraft are prohibited from operating faster than 250 knots Indicated Airspeed below 10,000 feet MSL in most cases. You would have to get special permission from FAA leadership (not ATC controllers) to otherwise perform such a stunt.

§91.117 Aircraft speed.
(a) Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).

At those speeds, you can hear the aircraft coming. But, the sound seems to be much more intense and at a lower (bass) frequency after it has passed rather than when it is approaching. Like hearing a race car at full-tilt passing you on the sidelines. Much more rumble-like after passing due in part to Doppler effect and the aircraft being closer to the ground when landing.
Speaking of closer to the ground, modern Noise Abatement Procedures have made it so large turbine powered aircraft avoid overflying populated areas at low altitudes. The aircraft will climb quicker to altitude, descend later from altitude, or turn away sooner, thereby reducing the effect of aircraft noise in neighboring communities.
The FAA and ICAO has taken further steps to reduce the actual noise produced by aircraft according to FAA.gov,

“In 1990, Congress passed the Aviation Noise and Capacity Act, which required that by the year 2000 all jet and large turboprop aircraft at civilian airports be Stage 3.”

Furthermore, FAA.gov goes on to say!

”The FAA has undertaken a phase out of older, noisier civil aircraft, resulting in some stages of aircraft no longer being in the fleet. Currently within the contiguous US, civil jet aircraft over 75,000 pounds maximum take-off weight must meet Stage 3 and Stage 4 to fly. In addition, aircraft at or under 75,000 pounds maximum take-off weight must meet Stage 2, 3, or 4 to operate within the U.S. In addition, by December 31, 2015, all civil jet aircraft, regardless of weight must meet Stage 3 or Stage 4 to fly within the contiguous U.S. Both Stage 1 and Stage 2 helicopters are allowed to fly within the U.S.”

Small military jets (not transport or cargo) sometimes operate at higher speeds along specific routes or in specific areas. Though, even at subsonic speeds, the effect can be quite jarring. When close to the ground, it is almost like a fighter jet is sneaking up on you in silent mode until it passes you in a great and sudden cacophony of sound. You don’t hear it coming, then BOOM! Unfortunately, that is still not a sonic boom. Just aircraft noise with a perceived increase in intensity as it thunders past you.
In the case of engine noise, you will continue to hear the noise as the aircraft fades into the distance. Like rolling thunder instead of the bomb-like or cannon-like dull thud of an explosion that is a sonic boom.

Actually you can hear those fighter jets coming.  I can't say that it NEVER happens that one "sneaks up" on you, but I get overflown several times a year (not fun when you're on a nervous horse!) and have always been able to hear them at least far enough in advance to say "Oh, crap!".

@jamesqf - That really depends on the speed and the altitude. I have been at sporting events, airports, airfields, air shows, large lakes, Gulf of Mexico, and military bases when a fighter jock has decided to make a low-level, high-speed flyby. Whether it is the backside of Fort Hood or the city side of Soldier Field, a fighter staying barely subsonic less than 1000 feet over your head can sneak up on you. Heck, you would be surprised how well a hovering Longbow behind a berm can blend into the background noise. I’ve heard GAU-8s on the range without ever hearing the airplane.

@jamesqf In the UK, some of the more mountainous regions are regularly used for low-level flight training.  One day, I and a friend were standing on top of a small hill in the Lake District to observe a partial solar eclipse.  Groups of Tornadoes were flying past at regular intervals, and we tried to spot them all while waiting.  In one group, we spotted one of a pair against a hillside, then tried to find its wingman; I saw it coming directly at us while my friend was staring in the wrong direction.  It was silent until about a second before it went overhead…  then ROAR.

@Dean F.: "less than 1000 feet over your head"? Knock a zero off that, and you'd be close to what some of them do around here.  One place I go a lot has long, narrow meadows running down to a lake, and I've seen them following the meadows, lower than the trees to either side.

I agree with your answer, @DeanF., however the things you're quoting all seem to be in effect from 1990 onward, while the OP mentioned a 1975-80 time frame. I agree that it absolutely was not commercial airline traffic creating sonic booms, but references of regs from the late 70s would make a strong case stronger.

@ Niels Nielsen I looked up F-22 rafter.  Definitely not what I saw!  They looked like commercial jets--very big & white.

If you lived near an international airport with commercial traffic, then it couldn't possibly have been sonic booms, as others have noted. What you likely have heard is jet noise.
Jet noise has been known to be notoriously bad. If you google sound scale, jet noise is pretty much always near the top. That's one of the primary reasons why city airports haven't flourished as well as people have thought back in the 70s. That being said, leaps and bounds have been made since the first commercial jet airplanes.

Ref: https://aqrc.ucdavis.edu/sites/g/files/dgvnsk1671/files/inline-files/UC%20Davis%20Noise%20101_0.pdf
The above chart shows the sideline noise of narrow-body jet (including turbojet and turbofan) powered airplanes measured in EPNdB, which is a scale for Perceived Noisiness Level. As you can see, there has been 15-20dB reduction between what's certified in the mid-90s and 70s. A 20dB reduction corresponds to a whopping 99% reduction in sound energy, and a 75% reduction in terms of hearing perceptibility.
Much of the noise reduction has been through the engines themselves, as shown below. For new aircraft, we've reached a point where airframe noise is comparable or larger than engine noise for noise testing.

Ref: https://aqrc.ucdavis.edu/sites/g/files/dgvnsk1671/files/inline-files/UC%20Davis%20Noise%20101_0.pdf
Note: Concorde SST is a definite abheration. It's certified in the late 80s and has a sideline noise of 112 EPNdB. That's when its speed is subsonic.

I note concorde wasn't on your sound scale, probably because they would have needed to make the y-axis logarithmic

@jamiec dB is already a logarithmic unit

Lol, true! Forgot

Sorry 4 delay in checking back 4 answers- didn't expect so many! Thanks every1! (1st time here).To answer people's questions, I was living in Barkhamsted, CT-small town in NW CT not far from MA border. It is in mountainous area--very close to a ski mntn. I recall 1 time very clearly-certainly looked like a commercial jet to me. I'd hear them coming but then just as it would fly directly overhead (low), I'd hear a deafening, cracking BOOM that I could feel in my body. Always made me jump. I recall my older brother saying "sonic boom". I've seen many fighter jets. These were much bigger & white.

You may have been hearing the Concorde reaching supersonic speed after its takeoff from the JFK and Dulles airports.
I heard them frequently while on Cape Cod during the summer months. Interesting studies were done about this phenomenon and its effect on the population.

Concorde is(was) only allowed supersonic over water : but I'm not clear how far offshore it had to be. Boom may have been audible for a few tens of miles...

And Bradley Airport is 80km from the nearest sea (Long Island Sound) and about 200km from Cape Cod (further than Boston is), so if Concorde was conspicuous there it would have been apparent to a large population.

Now the Concorde idea is interesting.  I was only a kid, & I just remember "very big & white" as the physical description.  & when I looked it up on Wikipedia, there's a Concorde that flew from 1977-1982 that doesn't give a location, except where it was scrapped more than a decade after its last flight (which is curious). Perhaps this Concorde was what I used to see: "211 F-BVFD[27] 10 February 1977 27 May 1982 5814 Scrapped in 1994. A small section of the fuselage remains at Le Bourget, France and the nose cone was sold to an American collector" (Wikipedia)

It was always going Southwest, btw.

What would real sonic booms have been like?
The other answers do a good job of explaining how commercial airliners have gotten quieter over the years and how you were probably hearing jet noise, not sonic booms. But how do you know it wasn't a sonic boom? What would it have been like to live under regular sonic booms? Well the only way to know that for sure would be to regularly fly supersonic flights over a heavily populated area and then study how people responded. Doing that would be a bad idea at best and madness at worst. So of course it was tried.
Oklahoma City volunteered to be the test city for a 1964 test. For six months, the Air Force flew supersonic flights over the city to see how much it annoyed residents. It wasn't subtle. The final report tried its best to put a positive spin on the results, but there was no hiding that there was a fundamental difference between jet noise and a sonic boom. While 70% of residents reported hearing airplane noise, 99% reported hearing sonic booms (Table 78 of the final report). And while 6% of residents reported being annoyed by airplane noise, 25% said they were bothered by the sonic booms (Table 79).

Over 80% of all respondents said they could always distinguish a sonic boom from other noises... Most of the people who can t always recognize a boom said they thought it was either an explosion or a thunder storm. It is interesting to note that
the distant area residents most often failed to recognize the boom and wondered if it was a storm or explosion

So if the sound you heard sounded like an airplane, it probably wasn't a sonic boom. You'd expect to hear something more like thunder or an explosion.
A whopping 40% of area residents reported damage to their homes. In one of the report's most optimistic sections, it tries to put a positive spin on the damage claims:

Direct scientific evidence indicates that the Oklahoma City booms did
not cause any significant damage to the local test houses, which were
instrumented by the FAA to measure physical effects of booms. Large
numbers of residents, however, felt their houses had been damaged.
Over 40% overall felt this way; while 50% of the annoyed and 86% of
the actual complainers also felt this way. This clearly suggests that
belief in alleged damage increased annoyance and complaint activity.

So even through the positive spin of the report, you see a picture of people who endured sonic booms that they thought were explosions and that they said caused damage to their homes. It sounds like what you heard wasn't quite this intense and was more likely to be jet noise.

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Is PVC tubing a good material for helicopter airframes?
Been thinking of getting into the hobby of building ultralight helicopters, and would PVC tubing be a good choice to build the overall airframe?

Not only is the strength to weight ratio poor, PVC also has a rather low modulus, so the airframe will resonate at a multitude of frequencies. While tolerable in a glider, in a helicopter this is unwise. The resonance with all that is vibrating in a helicopter will shake the airframe apart before first flight.

No. The strength to weight ratio of PVC is poor compared to aluminum and you would end up with a very heavy, albeit cheap, machine, and will be even worse when compared to carbon fibre tubing, which would be the optimal choice performance-wise.  Extruded aluminum tubing would likely be the best choice cost wise.  Forget about building a plumb-O-copter.

If you're talking about the sort of PVC tubing used for irrigation systems, not only is it not very strong, it's also quite flexible.  Hold a 10-ft length (standard size in US hardware stores) in the middle, and it will deflect at least 6" on either end.

I think he has in mind Sched 40 or Sched 80 rigid plumbing pipe.

If I'm not mistaken (I'm no expert), schedule 40 PVC is the white pipe commonly used for irrigation &c.

Sched 40 is the standard PVC for residential house plumbing, good for a 2 or 3 hundred psi burst.  Schedule 80 is for higher pressure applications and has nearly double the wall thickness (smaller ID).

Sure, but bursting strength is not really what's important in building airframes :-)

I'm just describing what sched 40 and 80 are. If I was going to build something structural that was non-helicopter-y  from PVC from Home Depot, that would be it.

About the only thing structural I'd build with PVC is the frame that keeps the (very light) bird netting off my strawberries :-)

In my experience using PVC to build scale models, PVC doesn't make a good construction material:

it's too flexible. PVC pipe will bend under its own weight.
the glue typically used to join PVC is pretty weak, impact loads will break the bond.

PVC is no where near stiff enough for a given weight. And its strength to weight ratio is poor. The structural efficiency of air machines is absolutely critical otherwise you are building a machine that may not fly at all.
Plus, do a search for helicopter ground resonance, especially watch some youtube videos of helicopters destroying themselves.

Ground resonance only comes into play when the helicopter has a rough landing. With PVC tubing it will resonate so much that it will be shaken to pieces even before first flight.

I'm pretty sure I have seen videos of ground resonance without a preceding landing of any sort

PVC pipes are strong and flexible. 1 1/2" diameter pipe type SDR-26 is thinner-walled and weighs only .3 pounds per foot! That's three pounds per ten-foot length which only costs eight dollars each at Home Depot. PVC cement can form a solid integrated bond when applied thickly and left on for ten seconds to melt the surface before mating.
However, PVC is breakable when bent, so the airframe must have a lot of reinforcement braces to reduce the amount of bending of longer sections.
Strength-TEST.!!  Test test, the strength of every section using sandbags. Get an EAA member to advise you on this.
Regarding self-bendability: I just tested a 10' length of 1 1/2" PVC type SDR-26 plastic pipe and when suspended it at center and each end drooped down 1/2" is all. I doubt that any ultralight designer would incorporate any unbraced pipe lengths longer than 2'.

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What is the engine shell temperature?
What are the highest temperature locations in the outer shell on an aircraft engine? Particularly, modern turbofans like the Trent 900 for example. I'm interested in the temperature constraints of pylons that's why.

By "engine shell" do you mean the nacelle?

No, the shell of the bare engine. The part where the engine mounts are installed (Inside the nacelle).

Not the combustion cans ?


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How hot is the outer casing of a turbofan engine?
I am able to find temperatures inside different components of turbofan engines, but I can't seem to find what would be the temperature of the external casing/shell of the engine. For example, temperature of the casing around the LPC, HPC, HPT, LPT etc.
Information related to any type of turbofan engine would be appreciated. Thanks.

Tough question to answer. Because the outer casing is exposed to the elements, the temperature will vary by a tremendous amount. Temperature at cruising altitude can get down to -57C (-70F). At that temperature and a cruising speed of 450 knots, the outer casing would be rather cold. If you are talking about an aircraft sitting on the ramp, the engine will not be generating the same thrust as it would in flight, therefore, less heat. Is it raining or snowing? Winter in Alberta or summer in Florida? There are many variables.

He might mean the outer casing of the core, which might heat the bypass air slightly.

The core of a TF has its own cowl. The bypass air flows past the core cowl so the heat picked up would be insignificant.  The hottest area is the turbine casing and tail pipe where you normally have overheat warning loops for fire detection.  Forward of that, a lot of the heat under the core cowl is coming from the bleed plumbing.  To quantify, you would need an under-cowl temperature study for a particular engine that was the basis for the engine's overheat detection system, and for that you'd have to try to get something from a manufacturer's engineering organization.

Thanks guys.
I'm asking about the casing (inside the cowling) of hot components of the engine such as the HPC and HPT in maximum operating conditions. As for as the particular engine, I'd like to know about Rolls Royce's Trent 900.
Information is difficult to obtain from online sources so if you guys can help out that would be very appreciated. Thanks.


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What is the temperature range of engine mounts connected to the pylon?
I want to know what is the temperature of the engine mounts in modern turbofan engines for example, Rolls Royce Trent 900. Also, what are the temperature considerations you need to make while designing a pylon?

This is an order of magnitude educated guess:
Based on this picture from Wikipedia:

You can see that the aft mount is attached to the turbine rear frame.  As such, it'll get relatively toasty.  I'd say maybe 900 - 1000 F.
The forward mount is attached to the fan frame just above the compressor case.  As such, it will be much cooler.  Maybe 200 - 300 F.
Again, that's just an educated guess.  I'm not aware of any publicly available data.
Edit:
in response to the comment, no it's not the big red box at the front. That's approximately where the forward mount would be in a typical Boeing style engine, but Airbus does it a little differently.  In the below, I found an image of a model of the pylon (from here), and showed where the pylon meets up with the engine.  The left most line points to the aft mount, the middle line to the forward mount, and the forward line meets up with the inlet (not a mount, just to show you how it lines up).  The top orange bar in the engine picture, and the lower white bar in the pylon picture are ground support equipment.  Would be removed when the engine is attached to the pylon.

Actually, I can't see where the fore and aft mounts are. Could you edit a description in of what I should be looking for? Is it the big red box up front and where the orange bars meet at the rear?

@FreeMan: "I can't see where the fore and aft mounts are", perhaps more visible here :-)

@mins, very good post, I upvoted, although that's mostly describing Boeing mounts, and OP here asked about Trent 900 which is on the A380. Aft mounts are fairly similar, but Airbus has a slightly different mounting style for the forward mount.


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What safety precautions are taken for an airline prototype test flight?
Regarding the software, the equipment and the personnel.
Can the crew jump out of the plane with parachutes or is it a bad idea?
Additionally, if you would compare the safety of today and the 1970/80s what would be the difference?
Thanks in advance.

Related/Duplicate (regarding parachutes): https://aviation.stackexchange.com/questions/7736/do-test-pilots-on-commercial-aircraft-wear-a-parachute

The bail-out procedure is usually only available on the first and second prototypes, which are the riskiest because they are usually designated for testing aerodynamics, dynamics, performance and flight controls, as well as for envelope expansion. Obviously, it's only used as a last resort when recovery is deemed impossible.
In an efficient and professional flight test organization, each flight test is planned out in advance. The test requirements and procedures are written by engineers to ensure that they can be carried out successfully and safely, with clear pass/fail criteria, as well as abort criteria should airplane deviate from safety thresholds.
The crew, including pilots, flight test engineers, and ground telemetry engineers, are briefed on the test procedures and the applicable flight envelope and systems restrictions on the test day. Flight risks are identified and treated accordingly. For example, if a stall test is planned, the risk to spin/deep stall may be high enough to warrant the arming of the spin chute.
The restrictions are some of the most critical technical documents to ensure the safety of flight. They contain the following:

Limit the flight envelopes, and expand only insofar as the planned procedures can be carried out successfully;
Outline systems limitations and defects, and procedures to go with them;
Performance penalties, including takeoff/landing speeds/distances, that may be impacted by the non-conformant systems or failures;

Any non-remote failure or combination of failures would have been checked out in a pilot-in-the-loop simulator with the best-known flight dynamics and systems models/rigs. The results of the evaluation would be a major driver in adding/revising/confirming the restrictions and the Airplane Flight Manual under development.
Another layer of defense lies in the onboard flight test engineers, who will be monitoring the critical flight parameters relevant to the test. They direct the flow of the testing and have direct line of communication with the pilots. For critical flights, ground telemetry personnel will also be monitoring the parameters. Should the parameters fall outside the abort thresholds, immediate abandonment of the test procedure will be called and the pilots will initiate recovery.


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Autorotation: Positive Pitch during descent phase possible?
I was wondering if a rotorcraft, that descends very slowly compared to its nominal rotor rotation rate (for example an RC Helicopter vertically autorotating with 3m/s descent rate and 2200RPM on the rotor) can maintain the rotor rotation rate even though it is having a positive collective pitch.
Is this possible? If so can it be sustained or is it just for short time frames?
The following image might enhance the understanding (Source)
look at the speeds and look at the lift/drag curve, if the lift/drag ratio is big enough this should work, right?

The usual pitch for autorotation in real-size rotating-wing aircraft is a positive +3º. Most gyros do happily fly with that fixed pitch in their rotors...  Anyway, the angle that matters is the angle of attack. In vertical autorotation, you can have a good AoA for autorotation even with a negative pitch, provided the vertical and circumferential airspeeds have the right value at the relevant station...

Thanks! That helps so much. I am working on a simulation for a Small Autorotating Reentry Vehicle that supposed to land on other planets (that's my Master Thesis) and I was confused that on Titan (very dense atmosphere, only 1/7 of earths gravity) I always run into positive angles of attack. It helps a lot to know that it can be possible!
I am not from the field of aerodynamics, I am a control engineer, so I really appreciate your input!

During autorotation there are two main angles which are important, the inflow angle created by the rate of descent flow and rotational airflow plus the pitch angle (normally minimal on initial entry to autorotation). It is these angles which produces the lift. The second angle produces the auto rotative force, or the force which maintains the rotor blades turning in auto. This is created by the forward tilt of the main lift vector (or the horizontal component of the lift vector) which is shown on your diagram. During autorotation it is important to maintain control over rotor rpm. Density altitude has a marked effect in auto, which is your consideration albeit to a greater extent. As density altitude increases rotor rpm for the same pitch angle will increase, this can’t be allowed to continue to increase and is contained by increasing blade pitch, hence the greater the DA, the more pitch is required including positive pitch if required. Another consideration which may help is, autorotation appears to be an aerodynamic effect of rotorcraft that is considered in isolation when a helicopter loses power, rather like an on/off switch. This isn’t the case, helicopters will experience a degree of autorotation in a normal descent due to the rate of descent flow but the rotor rpm will be maintained within normal limits due to the rotor governing system. It is a progressive process from the start of a descent to fully established auto when the torque to the rotors becomes zero.

I can see how you may have been confused. First para rewritten for clarity

The angle of attack ALPHAr is the key magnitude in autorotation. The following images are from Gessow & Myers 'Aerodynamics of the Helicopter'. The angle of attack ALPHAr = PHI+THETA, PHI being the inflow angle and THETA the pitch. It's easy to see that, if PHI is high enough, stable vertical autorotation may be achieved with zero (or even negative) pitch...

!]2

The Wells turbine, used in some wave power plants, is an example of autorotation with zero pitch:

Thanks!
I like how you say "even negative" I have only dealt with RC Helicopters and similarly sized vehicles and there the general rule of thumb  is that only negative pitch shall be applied for AuRo. This gives good perspective! I found myself sitting there and think that positive should be possible too, drawing exactly the image you have provided. But then I also thought "well years and years of RC Heli Piloting experience have to be worth something" so I listen to these people. But you guys are right, positive angles are fine as well its all about alpha

I don’t have the resources here to post a diagram but also explore conservation of angular momentum. In short if you are manoeuvring and increase the load factor on the disk it will flex up at the tips (similar to an aircraft wing) this in effect reduces the rotor diameter.  All things being equal the smaller diameter will have a higher rpm (think of an ice skater spinning that pulls her arms and legs in and spins faster). Often when turning this increase will require a collective increase to contain RRPM within limits.


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Sub-ideal Autorotation in Rotorcraft
I have another AuRo question for you.
When a Helicopter autorotates (ideally) it is in the turbulent wake state and there is virtually no flow through the rotor disk (figure of Leishman attached). Now, when Vc (descent speed) is roughly -1.8 times vh (induced velocity at hover) the vehicle is considered to autorotate ideally.
What if we do not autorotate ideally? Does the vehicle still have to be in turbulent wake state?
The real question: Is autorotation (and thereby maintaining of an ideal rotor rotation speed) possible in the windmill break state?
For instance, in the work of Dalamagkidis the OHA58 (Chapter 5.1.1) and the Raptor30v2 (Chapter 5.3.1) do not seem to be in turbulent wake state.
As for my calculations, vc/vh was always slightly lower than -2.
However, the vehicles seem to perform well and land softly.
So is the windmill break state a legitimate state for "autorotation"?
Thanks for helping me out! :)

Autorotation in helicopters is rarely ideal, with net rotor torque = 0, because some energy is drained from the turning blades to feed the tail rotor.
There's, however, a continuum between the ideal 'autorotation' and the 'windmill brake state/moulinet frein'. The 'point of ideal autorotation' should be taken as the 'point of minimum'. For any increase of the load (the weight of the gyro, for instance), from that point on, there's a multitude of possible equilibrium points in autorotation along that curve of 'windmill brake state'. Real-word autorotation is always in one of those points, and never at the 'point of ideal autorotation'...  You may find useful this illustration and text. Source included.

Given this is the first time I’ve encountered the mathematics associated with auto rotation and helicopters I find the chart to be a little vague. What I can say is that the descent in auto rotation and the round-out and touchdown are two different phases.  During stabilized autorotation descent, you’re trying to target a specific airspeed to maintain your rotor speed.  There will be upward airflow through the rotor disk; there has to be to continue to provide lift to the driving regions of the rotor disk and keep the rotor spinning.  I suspect that the chart indicates that the windmill brake state is a case where you do not have an effective equivalent relative wind over the blades and a large portion of the disc is stalled.  This would be very bad as the rotor can no longer maintain speed, making recovery impossible.  During the roundout, you are effectively exchanging rotor momentum for lift at slow airspeeds, which is why the timing and altitude of the roundout is critical to a safe touchdown.
I hope that helps.


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Autorotation Trajectory Datasets?
I was looking for works/datasets on autorotation trajectories. Now, I have found works like this: Aponso et al, Dalamagkidis et al, Abbeel et al  or Nonami et al (Chapter 6).
However, I have a hard time finding any real flight data of a helicopter during autorotation. Nothing, ranging from full-size helicopters to model helicopters.
I know it's a long shot, but are there any cases where the data would be publicly accessible? Datasets should include (Trajectory, Velocities, Attitude, Main Rotor RPM and if available blade pitch angles)?
I am running out of ideas. If there is no data I am going to get a model helicopter and do it myself :)

I do not know of any full-size helicopter datasets, but you might have some luck using the Px4 Flight Review site: https://logs.px4.io/browse which has publicly available data for drones. Perhaps you find someone who recorded his/her autorotation. Perhaps you can download and filter this data somehow...


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Data on Helicopter Moment of Inertia
I am looking for some data on real helicopters.
Specifically, something that is very hard to find is the moment of inertia of the main rotor.
I found the following:

CH-53 https://ntrs.nasa.gov/citations/19810003557
OH-58A https://digitalcommons.usf.edu/etd/1921/

I tried some flight handbooks and user manuals but with no success. I was also looking for rotor blade masses to estimate the MOI with. That also brought no success. Even ChatGPT was absolutely lost on this one. Mails to manufacturers where also sent a while ago, with no replies thus far.
Do you guys have any idea where I could find such data? One or two more helicopters would suffice for my cause.
Thanks!
PS: Is this some kind of extreme secret I am looking for here? Is it such a sensitive parameter? Or is it rather the opposite and "usually not interesting"? I have a hard time believing the latter because quite a few formulae need the Rotor Moment of Inertia for basic helicopter calculations.

Hi, Resource (including data) location, is off topic here. Take a look at [help/on-topic] for more info.

You are right to assume that it is a property which is well known by the manufacturer and also needed for a lot of modelling and simulation work. However, as all industries, the manufacturers are quite secretive about these kind of numbers and why wouldn't they be, after all you could be a competitor looking to gain an advantage.
Without contacts into the industry and an NDA, I doubt you will get the exact number, BUT you could try to estimate. Simply make a couple of informed assumptions about where the main masses sits, and you should arrive at reasonable numbers. Good luck!

You can estimate the weight using the last chapters of Johnson (pdf). Chapter 29.2 should be the right one

@sophit thanks for your reply. The problem is that I am building a model for estimating it and to verify that my estimates are good (enough) I would need real data. I am aware of the work you sent by Johson, however, they only estimate weights and not weight distribution in the blade, making an estimate for MOI hard.

Blade's structure is quite homogeneous along the span, you can simply consider the weight as being linearly distributed too.


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Is blade flapping "fully automatic"?
I have a rather simple question just to "double check" if I understood blade flapping right. So, I understand that a helicopter, in forward flight faces an asymmetric lift. Blade flapping is there to compensate this.
As far as I understand this, the higher lift on one side leads the rotor blade to tilt upwards (aka flap) thereby also tilting the lift vector and decreasing its purely vertical component. It flaps just enough so that in total it creates a symmetric lift situation.
Is that correct? All of this happens just by the nature of it? The pilot or flight controller don't have to do anything here?
My doubt here is that, if I apply an axial force on a object that rotates around a centre, it will reach its maximum "tilt" or in this case flapping angle, only 90° after the force was applied. This seems to contradict my understanding of blade flapping. So that’s why I am confused.
Context: I am an aerospace engineer (with emphasis on space) who tries to learn more about helicopters. My curriculum didn’t contain anything about helicopters.

Just to answer the part about the pilot's role in blade flapping, as a former helicopter pilot, the pilot doesn't have to do anything. The flapping occurs naturally, due to the nature of the assymetry of lift. The vertical movement of the blades "cancels" the assymetric lift that would otherwise be generated.

Not just flapping, there is lead and lag too.  Look for some videos shot from the root of a blade along its length as it spins around.  You'll find that the blade moves around a lot.

Blade flapping is indeed automatic but note that the cyclic control of the pilot will fine-tune it such that lift is equal on both sides of the rotor disk.
It is not the maximum upward travel of the blade or the tilting of the lift vector which reduces its lift, but the motion itself. If you look at the flow components, you get one from the circular motion of the blade ($\omega$ x r), a second one from the forward motion of the whole helicopter (v) and a third one from the upward motion of the blade (v$_z$). This upward motion reduces the angle of attack of the blade, and this reduction in angle of attack is what reduces blade lift.
Note that the high position of the blade at the end of the forward half of the rotor disk gives it an ideal position to increase angle of attack on the backward-moving side of the rotor disk by moving down again. This increases lift and is the other but equally important side of flapping.

v$\;:\;\;\;\;$Flow speed from the movement of the helicopter through air (light blue).
$\omega$ x r: Flow speed from the rotation of the rotor (dark blue).
v$_z:\;\;$ Vertical speed component from the up/down movement of the rotor blade (red)
The green arrow shows the resulting flow speed which is the vector sum of the speed components.

So if I understand correctly the upward and downward movement vz (due to flapping) is the key because it changes alpha just right to change lift. Thats.... insane! and very well explained! Thanks Peter :) Its so crazy that this "just happens" on its own.

@Clex I just added a first sentence to answer the question directly. Yes, flapping does most of the work but it is up to cyclic control to fine-tune the rotor so it produces equal lift on both sides.

@Clex: I've had this up for three days and nobody noted that the speeds on the backward-moving side were wrong. Fixed.

I saw it but didn't think much of it/was to afraid to ask. But I thought the advancing blade is moving upward and the "backward" moving blade is moving downward... now I am confused again.

Ah yes since the blade moves upwards the air, relative to it moves downards, and that is vz. Now I got it :)

@What I had to change was the direction of v in the backward moving blade. As it was, both blades would move forward, against the airflow, only one up and the other down. Now the backward moving blade is moving with the airflow so v gets subtracted from the circumferential speed. And yes, all arrows are supposed to show flow speeds, with the green arrow as the resulting speed.

@PeterKämpf: in you schematic you also forgot the induced velocity which is one of the most important and complicated velocity component of the rotating-blade world.

@sophit I wanted not to make it overly complex. It should show why forces vary between sides and does so without adding too much complexity.

@PeterKämpf understand and agree

(with my limited understanding) I do agree, however, in the easiest approximation, uniform induced flow, vi just adds another velocity component downwards, correct?

@Clex It is a bit more complicated, but in essence yes.

Blade flapping is both due to an "automatic" effect and a "controllable" effect.

The "automatic" effect is exactly as you've understood it. The advancing blade sees a total speed higher than the retracting one and therefore it creates a higher lift (and drag and pitching moment) than the retracting one. The opposite is true for the retracting balde. In order to compensate for this difference between the advancing and the retracting side of the rotor, the blades are left free to flap: flapping upward reduces the AoA seen by the blade and therefore the higher lift gets reduced as well. So the flapping is not only "automatic" but self stabilising too. This loop looks something like this:
higher speed on advancing blade $\Rightarrow$ higher lift $\Rightarrow$ flapping upward $\Rightarrow$ reduction of AoA $\Rightarrow$ decrease of lift.
The maximum flapping angle happens with a <90° delay: if the maximum lift happens when the blade is, say, on the right of the helicopter then the maximum flapping happens when the blade passes over the front of the fuselage (for a rotor rotating anticlockwise as seen from above).

The "controllable" effect is based on the same idea but controlled by the pilot via the "longitudinal" stick. If the pilot pushes the stick forward, the blade's AoA (aka feathering) is incremented when the blade passes over the left side so that its maximum flapping angle is reached when it passes over the tailboom. The total effect is that the rotor and its thrust tilt forward, just like commanded by the pilot.

The <90° delay between disk tilting and the feathering that causes it depends mainly on the distance of the flapping hinge from the rotor hub: for a teethering rotor the hinge is exactly by the rotor hub and the delay is 90°; for other rotor constructions the delay is smaller.
Note also that, albeit similar, this delay is caused by the fact that the feathering of the blade is in resonance with the flapping movement: so you don't have to think about someone pushing the border of a spinning top rather to someone pushing a swing


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Leaning Mixture Methods
I just started my PPL training. I was going through the procedures of leaning the mixture during cruise and found that different instructors use different methods.
My instructor asked me to lean the mixture using RPM method where she asked me to keep leaning the mixture till the till the RPM starts to increase to its peak and then stop as soon as it starts decreasing.
Example - if RPM goes from 2000 -> 2100 while leaning and then 2100 -> 2080, stop leaning and make the RPM 2100 for best results.
Meanwhile I have heard other instructors use EGT method which I am not sure of.
Can someone point out the differences and what is the actual reason behind both methods.
I train on the Piper Archer PA28-181 G1000
Thanks.

You are missing one. There is also leaning to fuel flow rate.

Leaning by exhaust gas temperature really only works with fuel injected engines, and you ideally want an exhaust gas temperature indication for every cylinder although it can be done with a single cylinder gauge. You normally EGT lean to the lean side of peak temperature, say 50-75 deg lean of peak, depending on the engine. With fuel injected engines, the mixture distribution to the cylinders is relatively even.
With a carbureted engine the mixture distribution is less even and one cylinder will reach peak EGT before the others and the EGT method is less effective.
You will notice the difference right away when you fly an injected engine. If you over lean, the carbureted engine starts to run rough as the first cylinder gets too lean to fire, while the others carry on as normal. The injected engine will generally just quit, as all 4 cylinders hit the over lean point at once (assuming the fuel distributor is working properly).
Leaning a carbureted engine can be done by leaning to best power (peak rpm) and enriching slightly. Best power will produce the highest cylinder head temperature and I personally don't recommend it for cruise. A bit rich of best power is better. Another method is to just lean to roughness, and enrich a bit (it's what I normally do).

The EGT method requires an EGT gauge, which not all training planes possess. The older the plane, the less likely it is to have one.


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How reliable are the available airfoil data sources?
I am making calculations based on Blade Element (Momentum) theory (BEM) for propellers and I am trying to find the angle of attack at which the propeller performs most efficiently.
If I have understood the theory correctly, the efficiency greatly depends on blade angle the instream velocity (and consequently phi) and the lift drag ratio.
A higher L/D results in higher efficiency.
But I have a hard time determine which of the Cl and Cd plots from different sources is reliable enough for my purpose.
As an example in below image I am showing three different plots for the same airfoil (RAF6) (Red, green and blue).
The problem as you can see is that they are all very very different.
I am comparing the green curve from the original plot (thinkness ratio 0.1) with airfoil data from airfoiltools (thinkness ratio 0.1 as well) and I also sketched a blue line that I sketched from an older NACA 650 document (pg. 28).
The vertical red line indicates the angle of attack the max Cl/Cd corresponding to the one from airfoiltools.

Questions:

Which of the three airfoil data sourced do you deem the most reliable?
Does anyone know where the one from wiki (reference below) is coming from (It would be nice to know which Re number applies to them)?
Are there any other (more reliable) sources for airfoil date (in this case for the puropse of BEM theory)?
Is data from airfoiltools actually reliable?
What is the highest L/D ratio ever measured (or what is the most efficient propeller ever)?
(And how does this compare to L/D ratios from propellers from the early days of aviation)

Notes:
Cl/Cd against airfoil plot from airfoiltools:
I concluded that I should be calculating with the result velocity (Vr) when calculating the Reynolds number for a propeller so I got Re numbers of approx. 2,000,000 and consulted the raf6-il from airfoiltools with the highest available Re of 1,000,000 with Ncrit=9 and 5.
Attribution:
Black and while Cl, Cd-plot coming from: https://en.wikipedia.org/wiki/Blade_element_theory
Marino108LFS, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons


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Time to take the ATPL and license conversion
How long does it take to get the EASA ATPL license on average?
And if, after obtaining the ATPL licence in a particular country, you are recruited in a foreign country, do you need to re-take examinations in order to be able to convert it into a valid one in that country, or is the one obtained directly in your own country OK? For example, if you switch from an EASA license to a FAA. Thank you


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Can i use diverter less intake in design based on MiG-29?
Which type of boundary layer diverter has been used in MiG-29?
Also, can I use diverterless inlet in my design that is based on the reference aircraft MiG-29?

You can use anything you want as long as you justify it. You are engineer, you decide.

You are right. Can you please tell me what originally have been used in MiG-29?


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Why did the RAF buy the F-35B model and not the A model
If the only issue is the in-flight refueling system (probe/drogue), wouldn't it make more sense to buy the A model and modify the tankers. The refueling issue doesn't compare to the performance penalty (range/internal weapons) of the B model.

It's only a guess, but maybe the RAF is replacing their Harriers and wanted VTOL ability in their replacement.

The most probable reason is because of commonality, to reduce costs and support logistics. But, it also increases operational flexibility. The UK MOD want to operate them off the Queen Elizabeth carriers, and they do not have a catapult launch, but a ski jump. Only the B version can get airborne with this setup. So, the RAF had to go with the B version also. As mentioned in this article, having all aircraft as B versions means it is easier to always have the amount needed to fill operational squadrons for the carriers. If you split the buy between A and B versions, A versions cannot fill the place of B versions down for maintenance. Note however, the decision wasn’t “plain sailing” (without debate), as mentioned by J Southwell’s answer.

That is for the Royal Navy. I was asking about the Royal Air Force. The conventional A model would have been much more suited.

@TejasTasgaonkar. Good point, I updated my answer.

The MOD originally planned to buy the B version, then changed their minds and wanted the C version, then changed their minds again and reverted to the B version when the cost of converting the Queen Elizabeth class aircraft carriers to catapult operation proved excessive.

Makes sense for the Navy. Was asking about the RAF.

The Navy operates ships - the RAF operate aircraft. So all the F35s will be operated by the RAF, and some will be based on-shore and some will be based on the QE.

@RAC That's not correct at all. The Royal Navy have the Fleet Air Arm, which will operate the F35 for the navy. Most RAF Pilots aren't trained in carrier operations - I'm not sure if this will change with the introduction of a common platform, but the two services are very much distinct and have their own pilots and aircraft. The Army have their Army Air Corps, too.

My understanding is that the intention on the part of the MOD is to have a fleet of F35B aircraft which will be capable of operating from either carriers or land bases as required.


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Why are fuel pump switches separate?
Since engines are run only when the fuel pumps are on and vice versa , why are there separate switches for this? Why can't it be automated ?

Since engines are run only when the fuel pumps are on and vice versa<<

Not true. Engines can be run without fuel pumps, to allow for fuel pump failure.
The need for fuel pumps depends on the location of the fuel tanks relative to the engines, and the relative thirstiness of the engines - there are some airplanes where the fuel pumps are only used at some times (i.e. near the ground).
So it all depends.

And some planes have electric fuel pumps that are only used for priming or when the engine-driven fuel pump fails, or when using fuels with low vapor pressure (e.g. mogas), etc.


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Induced drag - inverse proportion between induced drag and parasitic drag
I recently learned that the slower an aircraft is traveling, the higher the induced drag and the lower the parasitic drag. The parasitic drag is lower because the particles have less force against the aircraft (all types of parasite drag: form, skinfriction, interference, and ram). However, why is induced drag higher?
From what I understand, I thought that it would be low as well, because if the aircraft is moving slower that would mean that the air pressure would be less on the bottom of the planform, meaning the vortex created at the tip of the wing would be smaller (smaller difference between pressures) thus having less induced drag.
Can someone help explain to me why it is inversely proportional as such?


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What is this plane approaching an airport in Asia? (Also, what is the airport?)
The airport is in Asia, that is all I know.

Welcome to aviation.stackexchange.com!

This airport brings a whole new terror to touch'n'go landings

@Richard Not a touch and go, but missed approach below tree and roof top level through the village. Indeed a whole new terror.

The airport is Lukla - Tenzing-Hillary in Nepal.
The aircraft is a Eurocopter AS350 Écureuil helicopter. I found a picture of one that has the same panel layout.

Other than "tiny airstrip in a mountainous region of Asia" I had absolutely no reason to think "Lukla", yet that's what came to mind. Guess I was right. Time to quit for the day!!

For the uninitiated: Lukla is often seen as the gateway to Sagarmatha/Mount Everest and features an unconventional runway which slopes noticeably down with the slope that it's on, in addition to having a variable slope.

@Jules: Oh, and it's also impossible to go around, because of the mountain in the way.


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What are the VOR channels used for?
I'm looking at different VORs on a sectional chart and I see the frequencies but I also see a channel number associated with them as well. What are these channel numbers and how do you use them?

They are DME (TACAN) channels. If you look close the the VORs, you'll see that they are either a VOR/DME or a VORTAC station. DME is the distance portion of a TACAN. When added to a VOR, you have a VOR/DME. If a complete TACAN is co-located with the VOR it is identified as a VORTAC.
The military uses TACAN. If you were to have a TACAN receiver, you wuold tune it using the channel number.
DME is available to civilian users with a DME receiver. The DME channels are paired to VOR/ILS/VHF frequencies (It's a multi-page table in the DME standards.) For convenience, DME receivers are tuned using the paired frequency.
When you tune a VOR or ILS, that frequency is also sent to the DME automatically tuning it to the proper channel. They are channels because DME actually uses two frequencies, one for transmit and one for reply.
So, to answer the question: unless you have a TACAN receiver, you don't do anything with the channel number. They are on the chart for reference.

Do civilian GA aircraft usually have a TACAN receiver or is that usually a military thing?

Military.  TACtical Air Navigation system.

@morpheus305  No.  There's no real benefit to a civilian to have a TACAN receiver.  VORTACs provide nothing more than what you get with a VOR and DME. And there are a lot of places where there are VORs but no TACANs. Standalone TACAN ground stations are generally on military installations where you can't use them. The military uses them because the stations are much smaller than a VOR, so they can be located where VORs can't such as on a ship. TACAN also supports air-to-air mode which is commonly used by tankers.


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How does an aviation business get an AFTN address?
How does an aviation business get an AFTN address to receive FPL messages from Air Navigation Service Providers?


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How to avoid shock (particularly Oblique shock) inside Cd nozzle?
Using the Area ratio (A/A*) and for the desired exit Mach number I have designed a cd nozzle. The Inlet total pressure (Po) and the total temperature (To) is calculated using Isentropic relations for the optimum condition.
FLUENT CONDITIONS:
Solver type: Density-based, Absolute, 2d, Axisymmetric
Model: Energy On, Inviscid
Material: Air (Ideal gas)
Boundary condition:
Inlet- Pressure inlet- 792812.3356 Pa; Supersonic/Initial Gauge pressure: 790000 Pa; Total Temperature: 518.4 K
Outlet- Pressure outlet- 101325 Pa; Backflow total temperature: 288 K
Operating condition- 0 Pa
For this condition, I should get an ideal flow throughout the nozzle, but there's an oblique shock inside the cd nozzle. Is this a design flaw or any error in the ANSYS fluent conditions?
How to avoid Oblique shock inside CD nozzle?


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How would fighter jets make contact with a previously hijacked aircraft?
For example, a plane is hijacked but civilians regain control, they know how to change the plane's frequency but no procdeures - i.e. dont know guard frequencies. There are fighter jets alongside who do have contact with ATC. What could the jets/ATC do to let them know what frequency to use?

BTW, what you're talking about has happened, and the person at the controls wasn't completely unqualified.

Very similar, but no relation to terrorism.

Does the terrorism part really matter for the purpose of this question?

I don't think you will find a single commercial pilot anywhere in the world who does not know the guard frequency

related: What's the point of intercepting an aircraft?, Are military jets always scrambled when communication to a commercial aircraft is lost?

I think you may be interressed by the interception procedures (chapter 5, section 6 of this publication, jump to 5-6-13-b for the part related to your question). This FAA-procedure is similar to the one I'm currently learning in Europe.

Is your question restricted to airliners? for smaller aircraft, helicopter interception with written orders can be dispatch, as in this article (fr)

Mobile phone?  The fighter pilots could hold up a number to call

Welcome to aviation.SE! I'm not completely sure what you're asking. There are already well defined procedures for interceptions, including frequencies, and pilots know what they are. Are you asking specifically about a scenario where the pilots are incapacitated and the intercepting jets have to communicate directly with the passengers? If you're new to the site, the [tour] may be helpful.

Nothing. There is no possible world in which someone has the necessary training, knowledge, and experience to operate the complex communication equipment, yet doesn't know 121.5.

The premise of the question is that a passenger defeats the hijackers, takes over the controls, knows how to operate the radio, but doesn't know the guard frequency. I would argue that such a person doesn't exist. It is much more likely that they would forget how to operate the radio than what the guard frequency is. And even if what you say would happen, and they forget the guard frequency but still remember how to operate the radio, then they only need to look down, since there is a high chance that the standby radio is tuned to guard anyway.

The fighters may have the the guard frequency painted on them, as in this photo:

(source and more photos)
But as others have said, the likelihood of a person

Knowing how to tune an aviation radio,
Recognizing what "STBY 121.50" means, and
Not knowing in the first place that 121.5 is the guard frequency

seems vanishingly small.
Besides, unless there was some kind of struggle the radio should be still tuned to the last ATC frequency. If the aircraft is in the flight levels it should still be within radio coverage even if it's moved beyond the controller's sector boundary.

The reality of this situation is that there are no standards for dealing with a passenger flying an airliner.
We can all come up with ideas of what a fighter pilot may do, but this isn't something that's written down and trained for by anybody.


text:
What is the purpose of these "antennas" on the aileron and winglet (A320 Neo)
As title says, see attached picture:


text:
What flight conditions trigger the "altitude, altitude" callout on an A-10?
I saw this video that shows an A-10 mission in Afghanistan. At 4:31 and 6:08 the callout "Altitude, altitude" is audible.
I would like to know which flight conditions are required to trigger the callout.

Welcome to aviation.SE!

You would need a manual that describes the particular Ground Proximity Warning System parameters for the A-10.  It'll be something on the lines of radar altitude below a threshold value, with a descent rate above a threshold, and terrain closure rate (due to sloping ground) above a threshold.

My guess is that it was selected 8000 ft in the autopilot and when you leave 8000 for a lower level it triggers "altitude altitude". Both events in the video happeneded at that altitude and they were not the maximum risrky condition (altitude/vetical speed)

Some EGPWS have this callout meaning Flight below DH (Decision Height) with gear up or Low Altitude mode active (Honeywell MK XXII). However in the context of military aircraft it's rather one of the conditions described in NATOPS F/A-18 flight manual:

A secondary radar low altitude warning function enabled by entering the appropriate altitude, up to a maximum of 5,000 feet on the UFC (Upfront Control). The secondary radar low altitude warning provides a single voice alert warning “ALTITUDE, ALTITUDE” when the aircraft descends through the selected altitude. The warning can be disabled by entering zero feet as the stored altitude.

A barometric low altitude warning function enabled by entering the appropriate altitude, up to a maximum of 25,000 feet on the UFC. The barometric low altitude warning provides a single voice alert warning “ALTITUDE, ALTITUDE” when the aircraft descends through the selected altitude. The warning can be disabled by entering zero feet as the stored altitude. Power up with weight-on-wheels initializes the stored altitude to 5000 feet.


text:
What is the maximum pitch achievable in a medium-powered airplane?
One of the answers to this question proposes a maneuver in which the plane is pointed almost vertically. Obviously, an aircraft with a high thrust-to-weight ratio can achieve this simply by climbing, and most aircraft with thrust vectoring can vector the tail down to pitch up at extreme angles. However, I wonder about aircraft with more modest thrust-to-weight ratios (say, < 50%). Maximum elevator will cause the tail to drop and initiate a climb. I understand that the aircraft can pitch up faster than it will actually climb, but at some point, the elevator will lose control authority until the flight path gets closer to the pitch angle. I suspect this point is much, much less than vertical, and probably less than 45 degrees (I presume it would have to be a little less than the maximum elevator deflection). And since an underpowered aircraft will have a maximum climb angle much less than vertical, I question whether all aircraft are actually capable of pointing nose-up in flight, even momentarily (and without invoking something like an uncontrolled spin).
Perhaps some aircraft could use a combination of maximum elevator and an airbrake to achieve extreme pitch rotations, but I would think the airbrake would have to be in a particular position for this to have a helpful effect.

I'm going to assert that most aircraft are capable of momentary vertical climb, even ones with zero engine power.
Achieving vertical upwards pitch is not about engine power, it is about momentum. An aircraft can gather enough momentum to achieve vertical pitch by first diving to gain speed (= momentum) and then pulling up for a (brief) vertical climb.
Limitations to the stunt would be such as max G, Vne, Vma and others mentioned in Pilots Operating Handbook.
The case will of course be somewhat different if we are only to discuss motorized planes starting the maneuver from straight and level flight at, say, maximum speed.
Recovery from vertical climb is not dangerous as long as you have enough speed to maintain control of the aircraft when you initiate the exit of the vertical attitude.
You mentioned using air brakes to maximize rotational speed. That is not how it works. Air brakes in almost all cases also destroy lift from the main wing, thus diminishing positive rotational speed not adding to it (this would qpply to the traditional wing setup).

Here's an example of a glider doing a pure vertical climb as part of a hammerhead: https://www.youtube.com/watch?v=qJ5iyOELCZM. (Don't unmute.)

Ok, I buy your argument. For the airbrake case, I was thinking specifically of the F-15, which has a large brake on the top of the fuselage that rotates upwards. Depending on how quickly that can deploy, I imagined a possible scenario where a pilot could use maximum elevator plus the airbrake deployment to momentarily create increased pitch-up rotation. Of course, the F-15 has a high TtW ratio, so it wouldn't need to do this stunt in the first place.

I'm pretty sure the F-15 airbrake does not have a positive effect on rate of rotation. Depends on how it is place in relation to CoG, don't know exactly. Also, at high rotation rate, the AoA will also be high, and the airbrake will be mostly in the "wake" of the front fuselage.

@LawnmowerMan -- Comments deleted-- now addressed in new answer.

If your goal was to achieve maximum pitch pretty much any airbrake would be counterproductive.

I understand that the aircraft can pitch up faster than it will
actually climb

What you are describing here, is an increase in angle-of-attack.  Over any given time interval, if the pitch attitude has increased more than the climb angle has increased, this generally indicates that the pilot has moved the control stick or yoke aft to increase the wing's angle-of-attack.
During aerobatic maneuvering, the angle-of-attack will often be increased, but normally not extremely rapidly, and normally not to the point of reaching the stall angle-of-attack-- with the notable exception of snap rolls, spins, etc.  For non-stalled aerobatic maneuvers, variations in angle-of-attack are typically in the neighborhood of 10 degrees or less, while variations in the pitch attitude may be unlimited.
Note that if we start from straight-and-level flight, a very rapid, large increase in angle-of-attack will invariably impose a very large G-load (lift force) on the aircraft during the portion of the maneuver before the stall angle-of-attack is reached, because the airspeed will not have time to bleed down to a lower value appropriate to the new, increased A-o-A. The need to limit the G-load (lift force) to safe values, typically limits how rapidly we can safely increase the angle-of-attack.
So normally during aerobatic maneuvering, we do not "pitch up faster than" we can "actually climb" to any significant degree, except for maneuvers involving intentional stalls.
Your model of a pilot commanding pitch attitude changes which can't be adequately matched by changes in the direction of the flight path, resulting in very large changes in angle-of-attack, doesn't accurately reflect how aircraft are actually flown in non-stalled aerobatic maneuvers.
We've noted above that the rate at which we can safely increase the aircraft's angle-of-attack is constrained by the need to keep the G-loading within safe limits.  But this inherent limit on the rate of change of the angle-of-attack does not equate to a limit on the rate of change of the climb angle or pitch attitude.  During aerobatic maneuvering, the pitch attitude will typically change much more rapidly than the angle-of-attack.
Even in a low-powered or un-powered aircraft (e.g. sailplane, hang glider) it is quite possible for the direction of the flight path to pass through vertical into inverted flight, without the wing ever reaching the stall angle-of-attack.
In fact, it would hypothetically be possible to fly some sort of modified variation of a loop with the angle-of-attack remaining constant throughout the maneuver.  (The aircraft would have to be quite strong, as the G-load (lift force) at the beginning of the maneuver, when the airspeed is high, would be very high.)

but at some point, the elevator will lose control authority until the
flight path gets closer to the pitch angle

I'd suggest that this concept doesn't really make sense.  Yes, if you increase the angle-of-attack all the way to the point of the stall, control authority is affected somewhat, but normally the elevator continues to work.  (Granted, in cases of extreme post-stall maneuvering, where the angle-of-attack is far beyond stall and the aircraft is being controlled by thrust vectoring etc, the elevator may be ineffective, but that's outside of the available flight envelope of most of aircraft.)  As explained above, you seem to be envisioning a very rapid increase in angle-of-attack.  Note that during the (initial) portion of this maneuver where the wing is not stalled, this will impose a very high G-load (lift force), which will cause a rapid upward curvature of the flight path.
So basically it seems you are describing a poorly-flown entry into a loop, where the flight path initially curves upward but at some point the pilot applies too much back pressure and stalls the wing.
Fundamentally, you seem to be viewing the elevator as a pitch-attitude control, and you are recognizing that this eventually leads to changes in the direction of the flight path ("until the flight path gets closer to the pitch angle").  I'd suggest that you think of the elevator as an angle-of-attack control, and recognize that pitch attitude changes are the combined result of changes in the angle-of-attack and changes in the direction of the flight path. A rapid change in pitch attitude is not necessarily an indication that the angle-of-attack is rapidly increasing, i.e. that the rate of change of the direction of the flight path is somehow "lagging" behind the rate of change of pitch attitude.
For example, we could demonstrate a rapid increase in pitch attitude even in a completely unpowered aircraft, without even touching the control stick at all.  In such a maneuver there would be no reason to suspect that the angle-of-attack was increasing at all.  Example-- in a glider (sailplane), trim the elevator for flight at the best-glide airspeed and angle-of-attack, with the wings level.  Remove your hands from the control stick.  Use the rudder to slowly roll into a steep turn, which will cause the airspeed to slowly and smoothly rise.  Once the airspeed has stabilized at the higher value appropriate for the steep bank angle, use the rudder to briskly roll to wings-level, applying opposite rudder as needed to stop the roll and "freeze" the bank angle near horizontal.  Due to the retained excess airspeed-- which equates directly to excess lift--  the flight path will curve upward sharply -- but to a first approximation the angle-of-attack will remain constant throughout the maneuver.
As other answers have pointed out, with sufficient airspeed and appropriate control inputs, there's no issue with bringing the flight path all the way to the vertical for short periods of time, even in unpowered aircraft (e.g. sailplanes, hang gliders.)  Loops are routinely flown in such aircraft.

Perhaps some aircraft could use a combination of maximum elevator and
an airbrake to achieve extreme pitch rotations

If the aircraft is performing a "Cobra" maneuver where the pitch attitude is brought to (and beyond) vertical with little change in the direction of the flight path, then it's difficult to say how various airbrakes, spoilers, etc might help or hurt the maneuvering capability in any given aircraft.  However if the pitch attitude is being brought to vertical in the more conventional way-- by curving the flight path through vertical-- then according to Newton's laws, a force must be applied to make the flight path curve or bend -- because a change in the direction of the flight path is a form of acceleration.  And that force is Lift. If the airbrake or spoiler is located in a position where it disturbs the airflow over or under the wing, then that will reduce the lift coefficient. That's not going to help the pilot "pull G's" (apply extra lift force) and curve the flight path.
Note that at normal airspeeds, pitching maneuvers are not usually constrained by the total amount of pitch torque that is available. At normal airspeeds, the pilot usually has plenty of control authority to put the wing at any desired angle-of-attack, all the way up to the stall angle-of-attack, just using the elevator alone.
Rather, with a low-powered airplane or glider, the issue is simply making sure that the aircraft has sufficient initial airspeed so that the airspeed will not drop to an unacceptable level-- meaning that the wing's lift force drops to an inadequate level-- before the maneuver is complete.

All of which leads to a follow-on question-- during the "Cobra" maneuver, what exactly are the dynamics that allow the aircraft to get into a vertical pitch attitude without curving the flight path above horizontal?  Specifically during the initial stages of the pitch-up where the wing is presumably not yet stalled?  Is it just a matter of a very rapid increase in pitch attitude?  Grounds for a new ASE question...?

perhaps the aircraft could use a combination of maximum elevator and an  airbrake to achieve extreme pitch rotations.

Air brake?  Who needs an airbrake when you can use the whole plane for drag.  This is the secret to slipping.
The 3 keys to rapid change in direction for controlled sustained flight are lightness, strength, and power.
This is easiest seen with an overpowered, delta winged model, perhaps inspired by this Alexander Lippisch design flown by the Navy.
These maneuvers would put a pilot (and the plane) under severe G load.
For aircraft with a lower power to weight ratio, gravity is extremely useful to gain speed, but speed means a wider turn.
What you have correctly assumed is, that to turn (into the vertical), one wants less speed but enough power to avoid stalling.  This is why modern fighter jets are very low aspect ratio (for strength) and very overpowered.


text:
Why would short-haul flights be pressurised at a higher cabin pressure?
In the article here : https://abcnews.go.com/Health/Healthday/story?id=4507808&page=1 it is claimed that

Short-haul flights are usually pressurized at 5,000 to 6,000 feet while long-haul flights are closer to 8,000 feet, according to Thibeault, [M.D., medical advisor, IATA].

But why would this be? My understanding is that the cabin pressure is chosen as a balance of:

what the aircraft can withstand without slow structural deformations, so that it is functional for many years
what is most comfortable for the passengers
most fuel efficient- pumping in air requires some fuel.

With regards to comfort, I should think that low pressures take their toll after a few hours (I think I read 3-9 hours, for people developing altitude sickness symptoms at about 8500ft equivalent cabin pressure).
The only guesses I have at why short-haul flights can operate at higher cabin pressure is:

the aircraft body won't have as much structural change with a short-term pressure difference
the plane can 'retain in' air pressure without actively pumping it for some time on the order of hours/ half-hours, so it is not so energetically costly.

For anyone else who read the title wrong: it says "higher cabin PRESSURE" (= lower cabin altitude)

For the structural loading, the absolute cabin pressure is not as important as the pressure differential with respect to the outside pressure.
Short-haul flights typically operate at lower altitudes; either because they are operated by turboprop aircraft which have a lower service ceiling, or because there is simply not enough distance to climb to a high altitude and glide back to land. At lower altitudes, the outside air pressure is higher, and thus the cabin pressure can be higher without exceeding the maximum pressure differential the fuselage can handle.
Your other concerns are not really a factor. Only the pressurisation cycle count really matters to aluminium, not the total pressurised time duration. The fuel required for pressurising the aircraft is also not really a factor compared to the energy required to either heat or cool the air for passenger comfort.

The Boeing 777 is a long-haul aircraft that can cruise up to 43,000’, but not with a full load of passengers and fuel. At the beginning of a long-haul flight the typical cruise altitude is performance limited to about 29,000-31,000’. At these altitudes the cabin altitude will be only 5,000-6000’.

@MikeSowsun Exactly - it's the altitude that determines cabin pressure, not long- or short haul.

Thank you for your reply. What do you think is the 'long haul/short haul' round-about cut-off (I realise that it will be dependent on many factors)? Would 2 hour flights, say, generally be flying at lower altitude?

@Meep The long haul/short haul is from the older hub-spoke model, where jets would do long haul and turboprops short haul. This is still true in some places but in other places jets do flights under an hour at high altitude too, much shorter than some traditional short haul flights.


text:
What does SWES mean in SNOWTAM?
The resources explains that SWEN is the start for Snowtam. (Ref: Snowtam help ) But many snowtams I found starts with SWES which I could not find anything about. What is the difference? Can it have other values? Does snowtam always need to start with one of them?
Example Snowtam from Pilotweb
SWES0701 ESSB 12270841
(SNOWTAM 0701
A) ESSB
B) 12270841 C) 12 F) 4/4/4 G) 1/1/1 H) 5/5/5
N) G1 R S T Y Y1 Y2 Y3
Y4 Y5 YU YW/4/GOOD
R) 1 2 3 4 6 7 EAST NORTH/4/MEDIUM
T) RWY 12 BA 60/48/41. RWY 12 COVER 100 PERCENT. APRON SANDED. ICE
PATCHES ON APRON.)

Snowtams follow the ICAO Format found in ICAO Annex 15, Appendix 2
This says that the abbreviated heading starts with SW** where ** is the ICAO location identifier. The location identifier document, ICAO 7910 is a \$245 document so I can't link to it here, but all the Swedish airports start with location identifers that are "ES", hence any SNOWTAM you find from Sweden will be SWES reports. Reports for the United States typically start with "K" so they would be SWK*. Canada is "C", so SWC*.


text:
How to interpret the N and R sections in SNOWTAM
I am trying to decode this snowtam for 2 days now and I followed this Snowtam Guideline.
So N is taxiway state and multiple taxiway states can be combined. R is apron state and multiple apron states can be combined. But in the example below, I cannot see how it makes sense. For example in R, are 1,2,3,4,6,7,East,North all apron names?
SWES0698 ESSB 12261815 COR
(SNOWTAM 0698
A) ESSB
B) 12261815 C) 12 F) 1/1/1 G) XX/XX/XX H) 5/5/5
N) G1 R S T Y Y1 Y2 Y3 Y4 Y5 YU YW/1/GOOD.
R) 1 2 3 4 6 7 EAST NORTH/1/GOOD
T) RWY 12 BA 67/72/67. RWY 12 COVER 100 PERCENT. APRON SANDED.)
CREATED: 26 DEC 2019 18:32:00
SOURCE: EUECYIYN

Yes, those are all apron names. You can see where they are on the airport chart.


text:
How do I decode FAA SNOWTAMs?
New FAA Notam site marks certain notams as Snowtam and provides Runway Condition. I can decode E) part using FAA Runway Condition. But how can I decode the Q section. Is there a comprehensive source for decoding Notams?
FAA publishes same notam in 2 different formats. I am interested in decoding ICAO Format.
ICAO FORMAT:
12/057 NOTAMN  Q) ZBW/QMRXX/IV/NBO/A/000/999/4221N07100W005
  A) KBOS  B) 1912310152  C) 2001010152
  E) 09 FICON 5/5/5 100 PCT WET  OBS AT 1912310152.

DOMESTIC FORMAT:
!BOS 12/057 BOS RWY 09 FICON 5/5/5 100 PCT WET OBS AT 1912310152. 1912310152-2001010152

ICAO Annex 15

This is FAA's runway condition format, which is different than the one used in Europe i assume. So ICAO Annex 15 does not seem to fit into this.

So, a SNOWTAM actually has a different format, and your example is just a standard NOTAM in ICAO format.
12/057 NOTAMN  Q) ZBW/QMRXX/IV/NBO/A/000/999/4221N07100W005
  A) KBOS  B) 1912310152  C) 2001010152
  E) 09 FICON 5/5/5 100 PCT WET OBS AT 1912310152.

This decodes into the following:
Series and number:        12/057
Nature of the NOTAM:      New (N in the NOTAMN)

Q) Qualifier line:
   FIR:                   Boston FIR (ZBW)
   NOTAM Code:            Starts with Q and the following four letters:
                          Subject: Runway (MR)
                          Condition: Not Specified / Plain Language (XX)
   Traffic:               NOTAM issued for IFR (I) flights and VFR flights (V)
   Purpose:               NOTAM selected for immediate attention of flight crew members (N),
                          for PIB entry (B), and concerning flight operations (O)
   Scope:                 Aerodrome (A)
   Limits:                FL 000 to FL 999 (000/999)
   Geographical location: 42°21' N 071° 100' W (4221N07100W)
                          Operation radius of the NOTAM: 5 NM (005)

A) Aerodrome:             Boston Logan (KBOS)
B) From:                  01:52 UTC 31 December 2019 (191231 0152)
C) Until:                 01:52 UTC 01 January 2020 (200101 0152)
E) Text of the NOTAM:     Runway 09 Field Condition
                          Braking action 5/5/5 (good for each third of the runway),
                          Runway is 100% wet
                          Observed at 0152 UTC on 31 December 2019

We could dig into the ICAO documents to find the official guidance, but different countries implement it differently sometimes anyway.  Here's a good document that I found which breaks down the different codes.


text:
With current technology, why is there still increased separation between aircraft and delays in low visibility conditions?
Since most (if not all) commercial aircraft are equipped with inertial and satellite/GPS navigation, why is increased separation between aircraft necessary? Can't controllers immediately identify the position and velocity of every aircraft?

In good weather, airports can move lots of aircraft because the ATC separation standards are much less than the bad weather, poor visibility ATC separation standards.
“Current technology” really hasn’t changed much over the years as ground based ILS CAT II and CAT III are still the most accurate ways to get get aircraft on the ground in bad weather.
Most of the increased separation is due to the requirements to have uninterrupted ILS ground signals reaching the aircraft on approach. Aircraft waiting to take off must wait behind hold lines farther from the runway, and aircraft on final must be spaced farther apart so as to not interfere with the ILS signals.
Once on the ground, aircraft will often experience delays getting to the gate due to the difficulty of taxiing in near zero visibility. Even parking at the gate can often require special standards and procedures.

There's always the possibility of machine error, human mistakes, and "blind spots". These can be minimized but never entirely eliminated. Things like minimum separations are set to provide a margin of safety in case things go wrong.
For landings, it is also necessary to provide a plane that has just landed with enough time to clear the active runway before the next plane on approach puts its wheels down. This also figures into how far apart the planes get spaced while on approach.

Not to mention the lack of (ground) surveillance at some airports and ILS protection areas

absolutely... -NN


text:
What are the differences between a reserve parachute and a bailout parachute?
Glider and aerobatic pilots wear bailout parachutes. Skydivers wear reserve parachutes. Both have the identical goal of saving the pilot's bacon when the primary flight means fails.
However, the manufacturers of reserve parachutes don't tend to manufacture bailout chutes and vice versa.
Is there a material difference between the two-- regulatory, physical, load requirements, etc...--, or is it just that the markets are so well-defined that there's little cross-pollination?

Seems to me that the terminology is not that set in stone.  Would a "reserve parachute" apply to a ballistic (rocket-deployed) parachute on a lightsport plane or ultralight?  Would it apply to the parachute on my hang gliding harness, which is designed to be hand-deployed and to lower both me and the glider together?  Seems to me that "reserve parachute" is a very broad and loose term, however "bail-out parachute" seems narrower and would seem to apply to a parachute worn by a pilot (normally with no back-up) for emergency use via exiting the aircraft.

Assuming that you are comparing modern bailout parachute and skydiving reserve parachutes there are many differences of note. I have more knowledge of the latter and so my bias is in this favour.
Training.
A bailout/emergency pilot is rarely expected to be used and therefore a pilot is unlikely to have undertaken detailed practical training on its use. It is not uncommon as a passenger required to wear such a parachute, to be told how to exit the air craft and then deploy it only, with limited practical rehearsal. By contrast Skydivers perform many rehearsals as part of their training. Furthermore, modern day skydiving reserve parachutes, are similar to the design of the main parachute, so by virtue of being trained to ‘pilot’ the main, the skydiver will have considerable knowledge to apply to the piloting of the reserve, even if reserve use occurs on their first jump.
Opening characteristics
Square parachutes typically take longer to deploy than round ones. Additionally the round parachute opening characteristics are more consistent. These factors, make round parachutes more appropriate for ultra low level use as might be required in an emergency last resort scenario faced by a pilot. Conversely, most skydivers should face their problems at a higher deployment altitude >2500ft typically, where the opening characteristics are less pronounced. Don’t get me wrong - square parachutes can be opened at exceptionally low altitudes too, for example base jumping but the use case, equipment and prep are somewhat different.
Manoeuvrability
Skydivers use square parachutes for a multitude of reasons, but a key factor is manoeuvrability. It makes sense that a reserve parachute should be of similar performance to allow the user to land at a safest possible place of their choosing. Being able to steer provides an additional element of opportunity to avoid some obstacles assuming appropriate training, experience and presence of mind.
Physical differences
A modern skydiving reserve parachute is typically integrated into the same external container used for the main parachute for that sole purpose. Many bailout parachutes are integrated into harness systems which serve other purposes, inc restraint, comfort, buoyancy etc.
Aside from the obvious physical differences between the round and square parachute, there will be other features that differ a reserve parachute from a main one. Canopy shape will often differ to ensure a more consistent opening and docile handling characteristics, vs sportier main parachutes. Profile and number of cells are common changes.
A skydiving reserve is designed to be deployed independently of a main parachute, but to be compatible should an inadvertent or deliberate deployment of both occur. The reserve parachute will typically have shorter lines to allow (and encourage) both canopies to fly side by side if needed. There are other differences in how both are packed and deployed, again to enhance the opportunity for the reserve to deploy effectively, but there are many customisations based on risks associated with particular types of jump, that make this difficult to summarise.
Finally a reserve parachute does not have a user operated release method such as a capewell, as it is integrated into the load bearing structure of the skydiving container and the harness would need to be removed. By contrast some bailout parachutes allow disconnection to allow for uncontrollable hazardous landings such as into water, or in high wind situations where being dragged on the ground is more possible. E.g landing on roof of building.
Finally to answer your points more specifically.
Given the difference between the two types of parachute, there has been significant divergence between manufacturers, although (if memory serves there are some manufacturers who do both) but typically there is more in common with military parachuting and emergency parachute manufacturers.
There are some material physical differences, but these are less important than the characteristics above.

Welcome to Aviation.SE! Try using "##" command for headings. When writing any question/ answer/ comment, you can use the "help" icon to get help with formatting; doing so will be appreciated by the readers. Good day!

While the parachutes themselves may have quite similar specifications (deploy up to terminal velocity, quick opening, payload of one adult, descent rate <8m/s, preferably around 5.5m/s) the attachment to the user and the location of the pack on the wearer’s body are somewhat different.  Glider chutes used to be and perhaps still do double as seat padding.  I’d guess the reasons for different types coming from different sources might be (a) designing a harness and deployment system is a bigger job than designing a parachute, or (b) both types are made in a factory in Indonesia and badged according to the wishes of the importer.